During observation interval 1-4, Stacy stops engaging in self-stimulatory behavior. You record the absence of her self-stimulatory behavior by marking a minus sign “–” in interval 1-4. At the end of the five-minute observation period, you count the number of intervals in which the behavior occurred so you can calculate the percentage of intervals in which that behavior occurred. In this case, self-stimulatory behavior was recorded in 13 of the intervals. By dividing 13 by the total possible of 30 intervals, and then multiplying by 100, you determine that Stacey engaged in self-stimulatory behavior during 43% of intervals. There are several different types of time sampling procedures. These procedures differ based on how you decide to record the occurrence of the target behavior.
The most common forms of time sampling procedures include:
◖◖ Partial interval. The observer records the presence of the target behavior (with a “+”) if the behavior occurs at any point during the interval. The observer records the absence of the target behavior (with a “–”) if the behavior does not occur during the interval.
◖◖ Whole interval. The observer records the presence of the target behavior if the behavior occurs during the entire interval. The observer records the absence of the target behavior if the behavior does not occur throughout the entire interval.
◖◖ Momentary time sampling. The observer records the presence of the target behavior if the behavior occurs at the end of a specified interval. This means the target behavior is recorded only if it is present at the exact moment the interval ends (e.g., when the recording states “1-4”). Even if the behavior occurs at other times during the interval, if it does not occur at the exact moment when the interval ends, the observer records that the behavior was absent. There are advantages and disadvantages with each of these time sampling procedures. For example, momentary time sampling is much easier, but it may not accurately represent a target behavior. A student could spend much of her time engaged in inappropriate behaviors, yet no instances of problem behavior would be recorded because of the timing of her actions. On the other hand, partial interval recording may easily result in recordings of inappropriate behavior, but it might not be very sensitive to improvements because behaviors are recorded even if they are fleeting. There is no perfect data collection system! You simply need to consider these points to minimize your greatest concerns about the accuracy of the data.
77 } Evidence-based Practice and Autism in the Schools, 2nd Edition